Synopsis

Description

A function may be called with a varying number of arguments of varying
types.
The include file
<stdarg.h>
declares a type
va_list
and defines three macros for stepping through a list of arguments whose
number and types are not known to the called function.

The called function must declare an object of type
va_list
which is used by the macros
va_start(),
va_arg(),
and
va_end().

va_start()

The
va_start()
macro initializes
ap
for subsequent use by
va_arg()
and
va_end(),
and must be called first.

The argument
last
is the name of the last argument before the variable argument list, that is,
the last argument of which the calling function knows the type.

Because the address of this argument may be used in the
va_start()
macro, it should not be declared as a register variable,
or as a function or an array type.

va_arg()

The
va_arg()
macro expands to an expression that has the type and value of the next
argument in the call.
The argument
ap
is the
va_listap
initialized by
va_start().
Each call to
va_arg()
modifies
ap
so that the next call returns the next argument.
The argument
type
is a type name specified so that the type of a pointer to an object that
has the specified type can be obtained simply by adding a * to
type.

The first use of the
va_arg()
macro after that of the
va_start()
macro returns the argument after
last.
Successive invocations return the values of the remaining arguments.

If there is no next argument, or if
type
is not compatible with the type of the actual next argument (as promoted
according to the default argument promotions), random errors will occur.

If
ap
is passed to a function that uses
va_arg(ap,type),
then the value of
ap
is undefined after the return of that function.

va_end()

Each invocation of
va_start()
must be matched by a corresponding invocation of
va_end()
in the same function.
After the call
va_end(ap)
the variable
ap
is undefined.
Multiple traversals of the list, each
bracketed by
va_start()
and
va_end()
are possible.
va_end()
may be a macro or a function.

va_copy()

The
va_copy()
macro copies the (previously initialized) variable argument list
src
to
dest.
The behavior is as if
va_start()
were applied to
dest
with the same
last
argument, followed by the same number of
va_arg()
invocations that was used to reach the current state of
src.

An obvious implementation would have a
va_list
be a pointer to the stack frame of the variadic function.
In such a setup (by far the most common) there seems
nothing against an assignment

va_list aq = ap;

Unfortunately, there are also systems that make it an
array of pointers (of length 1), and there one needs

va_list aq;
*aq = *ap;

Finally, on systems where arguments are passed in registers,
it may be necessary for
va_start()
to allocate memory, store the arguments there, and also
an indication of which argument is next, so that
va_arg()
can step through the list.
Now
va_end()
can free the allocated memory again.
To accommodate this situation, C99 adds a macro
va_copy(),
so that the above assignment can be replaced by

va_list aq;
va_copy(aq, ap);
...
va_end(aq);

Each invocation of
va_copy()
must be matched by a corresponding invocation of
va_end()
in the same function.
Some systems that do not supply
va_copy()
have
__va_copy
instead, since that was the name used in the draft proposal.

On some systems,
va_end
contains a closing aq}aq matching a aq{aq in
va_start,
so that both macros must occur in the same function, and in a way
that allows this.

Bugs

Unlike the
varargs
macros, the
stdarg
macros do not permit programmers to code a function with no fixed
arguments.
This problem generates work mainly when converting
varargs
code to
stdarg
code, but it also creates difficulties for variadic functions that wish to
pass all of their arguments on to a function that takes a
va_list
argument, such as
vfprintf(3).

Example

The function
foo
takes a string of format characters and prints out the argument associated
with each format character based on the type.

Colophon

This page is part of release 3.80 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
http://www.kernel.org/doc/man-pages/.

License & Copyright

Copyright (c) 1990, 1991 The Regents of the University of California.
All rights reserved.
This code is derived from software contributed to Berkeley by
the American National Standards Committee X3, on Information
Processing Systems.
%%%LICENSE_START(BSD_4_CLAUSE_UCB)
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
3. All advertising materials mentioning features or use of this software
must display the following acknowledgement:
This product includes software developed by the University of
California, Berkeley and its contributors.
4. Neither the name of the University nor the names of its contributors
may be used to endorse or promote products derived from this software
without specific prior written permission.
THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.
%%%LICENSE_END
@(#)stdarg.3 6.8 (Berkeley) 6/29/91
Converted for Linux, Mon Nov 29 15:11:11 1993, faith@cs.unc.edu
Additions, 2001-10-14, aeb